1. Field of the Invention
This invention relates to printed circuit boards and, more particularly, to a u.v. curable solder mask coating over a substrate including a metal which has improved adhesion between the metal and mask.
2. Description of the Prior Art
Printed circuits are commonly based on rigid boards made of fiberglass impregnated with epoxy resins; or flexible boards which are based on sheet polyester and polyimides. Adhered to the board or sheet base is a metal layer, e.g. a copper layer--electrodeposited copper for rigid boards, rolled copper for flexible boards, on which the circuitry is printed and etched.
Coated over the board and metal is a solder mask polymeric coating which will mask the solder from attaching to the metal. Usually polymeric solder masks are thermally cured in order to provide adhesion between the mask and metal. However, recently, u.v. curable solder masks have been used for such printed circuit fabrication. Compared to thermally cured solder masks, u.v. curable solder masks do not have as good adhesion on metal substrates.
Baking a u.v. curable coating often can improve adhesion but this additional step is not desirable under production conditions. An alternative approach is to use adhesion promoters as additives in the u.v. circuit solder mask formulation. Unfortunately, adhesion polymers often have a significant impact on the shelf life, stability and/or overall performance of the u.v. curable solder mask. Particularly, such properties as electrical insulation resistance, hydrolytic stability, heat and chemical resistance and compatibility with the coating formulation may be affected by the presence of an adhesion promoter in the u.v. coating formulation.
Accordingly, it is an object of the present invention to provide a method of making a printed circuit board which includes a u.v. solder mask coating over a substrate including a metal in which the boards are characterized by excellent adhesion between mask and metal, particularly circuit boards which meet the IPC-SM-840A Class 3 standard for adhesion.
Another object of this invention is to provide such improved boards which otherwise retain all the advantageous properties and performance of the solder mask.